The thing I always do when I encounter a large code base, is roam though it and make a mental map of the structure; the good parts and ones that would benefit from some TLC.
A part I noticed did not yet receive the amount of attention it deserves are unit tests and asynchronous code handling.
I'll be blunt about it, here's what I did: I ported the async NodeJS library to a Mozilla-style module and on top of that I built a unit testing library, inspired by Mocha (among others).
I can imagine that the fellas I just mentioned don't make you recall anything, so here are two links:
First of all, the Github numbers will show you that they are crazy popular NodeJS libraries. And well-documented. And well-written. And awesome.